1. Field of the Invention
This invention relates to fluid flow control, particularly but not necessarily for use in metering apparatus for fluid.
2. Description of the Prior Art
EP-A-0280537 discloses a dosing system for filling containers with a particulate/liquid mixture. The system includes three single-acting piston-and-cylinder devices, of which a first device delivers the mixture downwards to containers advanced beneath it, a second device feeds a thick particulate/liquid mixture via a first conduit to the first device, and the third device feeds a thin liquid via a second conduit to a liquid supply port of the first device. The port is disposed peripherally in the cylinder of the first device and is swept by the piston thereof, and a conduit in continuous communication with the port extends through the piston to a lower axial end of the piston. One of the factors limiting the frequency of filling of the containers is the time taken to refill the second and third devices, which becomes relatively high for the second device with relatively viscous mixtures.
From GB-A-377939; GB-A-1305729; GB-A-1578981; FR-A-2068800; FR-B-2544491 and DE-B-1159341, for example, it is known to employ for metering purposes piston-and-cylinder arrangements which are double-acting in the sense that, while one metered dose is being delivered from one metering chamber, a second dose is being metered by another metering chamber and then, while that second dose is being delivered from the other chamber, a third dose is being metered by the one chamber. Of these, GB-A-377939; FR-A-2068800; FR-B-2544491 and DE-B-1159341 disclose that each arrangement includes a rotary change-over valve device which connects the fluid input to one metering chamber and the fluid output to the other metering chamber, and vice-versa, alternately. The valve device includes a cylindrical valve housing and a rotary closure member co-axially mounted in the housing. The housing is formed with four fixed ports, namely a fluid inlet port, a fluid outlet port, and two ports connected to the respective metering chambers. In FR-A-2068800; FR-B-2544491 and DE-B-1159341 the rotary valve device is disposed directly between the two metering chambers and the two pistons are disposed outwardly of the two chambers.
Cleaning of such apparatus in situ can be performed by treating a cleaning fluid as if it were the filling fluid(s) and thus causing it to flow through the apparatus along the path(s) of the filling fluid(s) by operating the whole of the filling system. However, with the known apparatus, the rate of flow therethrough would be relatively low, so that the metering chambers would be only poorly cleaned, to the extent that dismantling of the chambers could be necessary to clean them well. Moreover, the land(s) of the rotary valve device would be only poorly cleaned because they are in sealing contact with the valve housing throughout most of the normal operation of the filling system. Furthermore, if the cleaning fluid is hot, it may become cooled to an undesirable degree during the time taken for the normal reciprocatory operation of the metering arrangement.